I was having a look at the ObservableCollection code (thanks to the awsome .NET Reflector) and was surprised to find that the Add and Remove methods are not overriden. How then does ObservableCollection raise the PropertyChanged or CollectionChanged event to notify when something is added or removed?
It overrides a bunch of protected methods of the base Collection<T> class, e.g. InsertItem(int index, T item), RemoveItem(int index), etc.
These overrides specifically raise the events:
protected override void InsertItem(int index, T item)
{
this.CheckReentrancy();
base.InsertItem(index, item);
this.OnPropertyChanged("Count");
this.OnPropertyChanged("Item[]");
this.OnCollectionChanged(NotifyCollectionChangedAction.Add, item, index);
}
Related
Domain objects shouldn't have any dependencies, hence no dependency injection either. However, when dispatching domain events from within domain objects, I'll likely want to use a centralised EventDispatcher. How could I get hold of one?
I do not want to return a list of events to the caller, as I'd like them to remain opaque and guarantee their dispatch. Those events should only be consumed by other domain objects and services that need to enforce an eventual consistent constraint.
See Udi Dahan's domain events
Basically, you register one or more handlers for your domain events, then raise an event like this:
public class Customer
{
public void DoSomething()
{
DomainEvents.Raise(new CustomerBecamePreferred() { Customer = this });
}
}
And all the registered handler will be executed:
public void DoSomethingShouldMakeCustomerPreferred()
{
var c = new Customer();
Customer preferred = null;
DomainEvents.Register<CustomerBecamePreferred>(p => preferred = p.Customer);
c.DoSomething();
Assert(preferred == c && c.IsPreferred);
}
This is basically implementing Hollywood Principle (Don't call us, we will call you), as you don't call the event handler directly - instead the event handler(s) get executed when the event is raised.
I'll likely want to use a centralised EventDispatcher. How could I get hold of one?
Pass it in as an argument.
It probably won't look like an EventDispatcher, but instead like some Domain Service that describes the required capability in domain specific terms. When composing the application, you choose which implementation of the service to use.
You are asking to have it both ways. You either need to inject the dependency or invert control and let another object manager the interaction between Aggregate and EventDispatcher. I recommend keeping your Aggregates as simple as possible so that they are free of dependencies and remain testable as well.
The following code sample is very simple and would not be what you put into production, but illustrates how to design Aggregates free of dependencies without passing around a list of events outside of a context that needs them.
If your Aggregate has a list of events within it:
class MyAggregate
{
private List<IEvent> events = new List<IEvent>();
// ... Constructor and event sourcing?
public IEnumerable<IEvent> Events => events;
public string Name { get; private set; }
public void ChangeName(string name)
{
if (Name != name)
{
events.Add(new NameChanged(name);
}
}
}
Then you might have a handler that looks like:
public class MyHandler
{
private Repository repository;
// ... Constructor and dependency injection
public void Handle(object id, ChangeName cmd)
{
var agg = repository.Load(id);
agg.ChangeName(cmd.Name);
repository.Save(agg);
}
}
And a repository that looks like:
class Repository
{
private EventDispatcher dispatcher;
// ... Constructor and dependency injection
public void Save(MyAggregate agg)
{
foreach (var e in agg.Events)
{
dispatcher.Dispatch(e);
}
}
}
In Dart, is it possible for a function to have a prototype associated with it?
Example Javascript code:
doStuff.prototype.isDefined = true; //is there anything like Javascript's function prototypes in Dart?
function doStuff(){
console.log("The function doStuff was called!");
}
Is it possible to do the equivalent of this in Dart (i.e., create a list of properties for each function?)
Two things to address here:
First, Dart doesn't have prototypes or prototypal inheritance, and instead uses classical inheritance. Rather than a prototype, objects have a class, and instead of a prototype chain, objects have superclasses.
Second, for your specific case, I think we'd have to see more of what you need to do to figure out the idiomatic way to do it in Dart. It should soon be possible to emulate functions with objects so that you can invoke an object and still have state and other methods associated with it.
See this article for more: http://www.dartlang.org/articles/emulating-functions/
When that capability lands you'll be able to do this:
class DoStuff {
bool isDefined = true;
call() => print("The function doStuff was called!");
}
var doStuff = new DoStuff();
main() => doStuff();
Which works if you have a fixed set of metadata about your function that you need to keep track of. It's slightly different from JavaScript because each instance of the function in Dart will have its own state for isDefined. I'm not sure if it's possible or easy to get multiple instances of the function in JavasScript, but you might need to make isDefined static so that the value is shared across all instances.
Dart does not allow you to add or remove member variables from an instance of a class at runtime. Rewriting your example in Dart it might look something like this:
class doStuff {
bool isDefined;
doStuff() {
isDefined = true;
}
void stuff() {
print('The function stuff was called!');
}
}
main() {
new doStuff().stuff();
}
If you wanted to add a property bag to a class in Dart you would write:
class PropertyObject {
Map<String, Dynamic> properties;
PropertyObject() {
properties = new Map<String, Dynamic>();
}
Dynamic operator[](String K) => properties[K];
void operator[]=(String K, Dynamic V) => properties[K] = V;
}
main() {
PropertyObject bag = new PropertyObject();
bag['foo'] = 'world';
print('Hello ${bag['foo']}');
}
Note that you can't access map properties using the '.' operator.
im looking for a way to add an PropertyChangeEvent to an object that I have defined. The goal is to raise a change event when any of the property of the object is been changed.
so i can do something like the following
var newItem:MyObject = new MyObject();
newItem.addEventListener(event.PropertyChangeEvent, myO_PropertyChangeHandler);
class MyObject extends EventDispatcher
{
public function doSomething() :void
{
// change values, and dispatch event
dispatchEvent( PropertyChangeEvent.createUpdateEvent( this, "myProperty", oldValue, newValue ) );
}
}
If you can't extend EventDispatcher because your object extends something else, and if that super class isn't already a subtype of EventDispatcher or implements IEventDispatcher (which includes most types), you need to implement IEventDispatcher manually. See the help page for IEventDispatcher for example code on how you do that (i.e. with an internal EventDispatcher doing the actual job).
If I understand you correctly you're looking for the Bindable Meta tag.
I have a structuremap configuration that has me scratching my head. I have a concrete class that requires a interfaced ui element which requires an interfaced validation class. I want the outer concrete class to get the default ui element, but get a concrete-class-specific validation object. Something like this:
class MyView
{
IPrompt prompt
}
class GenericPrompt : IPrompt
{
IValidator validator
}
class MyValidator : IValidator
{
bool Validate() {}
}
How can I configure structuremap with the Registry DSL to only use MyValidator when creating dependencies for MyView. (And assumedly using BobsValidator when creating dependencies for BobsView)
Are you getting MyView (and BobsView) from the container? Can we assume that they will all take an instance of IPrompt?
One approach would be to register all of your validators with a name that matches the names of your view. You could implement your own type scanner that just removes the Validator suffix:
public class ValidatorScanner : ITypeScanner
{
public void Process(Type type, PluginGraph graph)
{
if (!typeof (IValidator).IsAssignableFrom(type)) return;
var validatorName = type.Name.Replace("Validator", "");
graph.AddType(typeof(IValidator), type, validatorName);
}
}
Now, if you assume an IPrompt will always be requested by a View that follows that naming convention, your registry could look like:
public class ValidatorRegistry : Registry
{
public ValidatorRegistry()
{
Scan(scan =>
{
scan.TheCallingAssembly();
scan.With<ValidatorScanner>();
});
ForRequestedType<IPrompt>().TheDefault.Is.ConstructedBy(ctx =>
{
var viewName = ctx.Root.RequestedType.Name.Replace("View", "");
ctx.RegisterDefault(typeof(IValidator), ctx.GetInstance<IValidator>(viewName));
return ctx.GetInstance<GenericPrompt>();
});
}
}
To retrieve your view with the appropriate validator, you would have to request the concrete type:
var view = container.GetInstance<MyView>();
Note that this will only work if you are retrieving your view with a direct call to the container (service location), since it depends on the "Root.RequestedType". Depending on how you plan to get your views, you might be able to walk up the BuildStack looking for a View (instead of assuming it is always Root).
When adding a new strongly typed view in ASP.NET MVC, these weird classes show up:
What are they?
Those are compiler-generated classes that handle closures. They're not unique to ASP.NET MVC.
This class will cause the compiler to generate one of these classes:
public class Foo
{
private bool _bar = true;
public Func<bool> HelloClosure()
{
return () => _bar;
}
}
When someone outside of Foo calls HelloClosure, they get back a function that has a link back to that particular Foo instance. Imagine we don't execute that function immediately and the GC comes along and collects Foo. Now what happens when we execute the function?
var fooInstance = new Foo();
var func = fooInstance.HelloClosure();
fooInstance = null;
GC.Collect();
// assuming fooInstance is collected
var result = func.Invoke();
These automatically-generated classes manage these dependencies between functions and instances so that we don't get into a situation like this.